Behavioral and Cognitive Equivalence Threshold: Foundations for the Practical Extension of Atkins and Roper

The United States Supreme Court's proportionality decisions in Atkins v. Virginia and Roper v. Simmons rest on a single constitutional principle: individuals who lack the behavioral and cognitive capacities necessary for full culpability cannot be sentenced to death. Contemporary neuroscience now provides the ability to measure these capacities directly. Research demonstrates that the neural systems supporting judgment, behavioral inhibition, emotional regulation, and future-oriented reasoning mature heterogeneously, vary substantially across individuals, and can be impaired by developmental deviation, psychiatric illness, traumatic injury, or neurodegenerative disease. Quantitative neuroimaging enables these impairments to be identified through norm-referenced structural and functional metrics, revealing when an individual's neural functioning falls below statistically defined thresholds. Behavioral and cognitive equivalence (BACE) operationalizes the Court's constitutional requirement by determining whether an individual's measurable functioning is equivalent to that of categorically exempt groups. Using validated neuroimaging techniques, normative modeling, and network-level analysis, BACE represents a transparent, reproducible method for assessing diminished capacity consistent with Hall v. Florida and Moore v. Texas. Integrating contemporary neuroscience with constitutional proportionality therefore supports extending categorical protection to individuals whose measurable impairments render them functionally incapable of the culpability required for capital punishment.